Skip MPI-CUDA demo when not building for the CUDA backend (#10)

Also remove unused variable in send_recv_usm.cpp

README.md

@@ -54,9 +54,10 @@ These three examples form part of the [Codeplay oneAPI for NVIDIA GPUs plugin
 documentation](https://developer.codeplay.com/products/oneapi/nvidia/latest/guides/MPI-guide).
 The documentation refers to the gpu-aware MPI guide for the CUDA backend.
 
-Building the MPI examples requires the MPI headers and library to be present on
-the system. This demo will be automatically skipped if MPI is not installed or
-detected and a message saying this will appear in the CMake configuration
+Building the MPI-CUDA examples requires the CUDA backend to be enabled and the
+MPI headers and library to be present on the system. This demo will be
+automatically skipped when not building for the CUDA backend or when MPI is not
+installed/detected. A message saying this will appear in the CMake configuration
 output. Additionally, in order to run the examples, the MPI implementation needs
 to be CUDA-aware. This is only detectable at runtime, so the examples may build
 fine but crash on execution if the linked MPI library isn't CUDA-aware.

src/MPI_for_CUDA_backend/CMakeLists.txt

@@ -1,11 +1,15 @@
-find_package(MPI)
-if(NOT MPI_FOUND)
-    message(STATUS "MPI not found, skipping the MPI_for_CUDA_backend demo")
+if(NOT ENABLE_CUDA)
+    message(STATUS "CUDA backend is disabled, skipping the MPI_for_CUDA_backend demo")
 else()
-    message(STATUS "Found MPI, configuring the MPI_for_CUDA_backend demo")
-    foreach(TARGET send_recv_usm send_recv_buff scatter_reduce_gather)
-        add_executable(${TARGET} ${TARGET}.cpp)
-        target_compile_options(${TARGET} PUBLIC ${SYCL_FLAGS} ${MPI_INCLUDE_DIRS})
-        target_link_options(${TARGET} PUBLIC ${SYCL_FLAGS} ${MPI_LIBRARIES})
-    endforeach()
+    find_package(MPI)
+    if(NOT MPI_FOUND)
+        message(STATUS "MPI not found, skipping the MPI_for_CUDA_backend demo")
+    else()
+        message(STATUS "Found MPI, configuring the MPI_for_CUDA_backend demo")
+        foreach(TARGET send_recv_usm send_recv_buff scatter_reduce_gather)
+            add_executable(${TARGET} ${TARGET}.cpp)
+            target_compile_options(${TARGET} PUBLIC ${SYCL_FLAGS} ${MPI_INCLUDE_DIRS})
+            target_link_options(${TARGET} PUBLIC ${SYCL_FLAGS} ${MPI_LIBRARIES})
+        endforeach()
+    endif()
 endif()

src/MPI_for_CUDA_backend/send_recv_usm.cpp

@@ -70,7 +70,6 @@ int main(int argc, char *argv[]) {
 
     q.submit(pf).wait();
 
-    MPI_Status status;
     // Send the data from rank 0 to rank 1.
     MPI_Send(devp, nsize, MPI_BYTE, 1, tag, MPI_COMM_WORLD);
     printf("Sent %d elements from %d to 1\n", nelem, rank);